Sildenafil citrate, a cGMP-specific phosphodiestrase type 5 (PDE-V) inhibitor, is the active constituent in one of the major treatments of erectile dysfunction. Recently, analogues conjugated with propoxyphenol showing greater PDE-V activity have been purified during drug discovery trials. Now these byproducts are turning up in health supplements marketed for sexual health enhancement. Although these products are readily available without prescription, they have not been through rigorous drug safety testing, and thus present major health concerns and even risk of death in users. In Singapore, the analogue propoxyphenylthiohydroxyhomosildenaﬁl (PP-THHS) has already been implicated in an adverse drug reaction that resulted in headache, giddiness, shortness of breath and backache in one subject.1

Kee et al. (2013) report on their success using high-resolution Orbitrap mass spectrometry (MS) to detect one of these so-called designer PDE-V inhibitors, propoxyphenylhomohydroxysildenaﬁl (PP-HHS), in a health supplement.2 Their method also accurately elucidated the structure of the new analogue.

HPLC-DAD screening of the extracted sample showed two peaks — one major peak corresponding to a thiosildenafil structure and one minor peak that also had sildenafil-like characteristics. These two peaks were purified and analyzed by Orbitrap MS.

The MS/MS spectra and proposed fragmentation pathways for each product differentiated the two sildenafil analogues. The major peak was similar to previously published fragmentation pathways and identified as PP-THHS, a sildenafil analogue with a recognized drug structure. The minor peak, however, presented a previously unknown analogue of sildenafil. By comparison with the known structure of the main peak and its fragmentation pattern, the molecular structure of the minor peak was identified as PP-HHS.

In this study, high-resolution Orbitrap MS successfully and accurately identified a potentially harmful contaminant in a commercial material available for use in health supplement production. The method shows potential for the screening of other products destined for human consumption and thus for protecting consumers from potentially fatal effects.